Organophotocatalyst Films and Their Multilayerization That Efficiently Utilize Natural Sunlight

A full-spectrum visible-light-responsive organophotocatalyst membrane array is designed and employed for a one-pass-flow water purification system.  Whereas previous photocatalyst systems required strong light source, the present design manages with natural sunlight intensity, owing to multi-layerization of a newly optimized low-absorbance organophotocatalyst.  The design of the system is to utilize natural-sunlight-equivalent visible light with 1 m² of irradiation area to process 1 ton/day of water.  A 1/3300 scale module of the system was constructed and experimentally demonstrated its viability.  The reactor part of the flow system contains 24 stacked layers of organic-semiconductor-laminated Nafion film.  The organic semiconductor is a bilayer of metal-free phthalocyanine (H₂Pc, p-type semiconductor) and 3,4,9,10-perylenetertacarboxylic-bisbenzimidazole (PTCBI, n-type semiconductor).  Transparent Nafion functions as mechanical support and absorbent of trimethylamine, which was chosen as a typical contaminant of underground water in coastal areas.  The reactor was irradiated for only 1 h/day by visible light (10 mW/cm²).  The light intensity at the bottom layer was estimated to be 0.1 mW/cm², which was sufficient intensity (Internal quantum efficiency was 0.15.) for the photocatalytic reaction, due to the optimized absorbance and photocatalytic quantum efficiency of each layer.  The inlet TMA concentration was 3 ppm, while that of the outlet was less than 0.03 ppm for the first day of the operation of the system with and without the bilayer.   Without the bilayer, the TMA concentration of the outlet flow increased after the 20 days.  With the bilayer, the TMA concentration of the outlet flow remained at less than 0.03 ppm for the 40-day experimental period due to its photocatalysis.  The turnover number of photocatalytic reaction was calculated to be 1.8×10⁴.

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